The present invention relates to a process for the preparation of highly reactive, low-viscosity and water-dilutable polyisocyanate reaction products which contain activated groups which react, by polymerization, with ethylenically unsaturated compounds under the action of actinic radiation. The present invention furthermore relates to a process for the preparation of such products and their use.
The curing of coating systems carrying activated double bonds by actinic radiation is known and is established in industry. Actinic radiation is understood as meaning electromagnetic, ionizing radiation, in particular electron beams, UV rays and visible light (Roche Lexikon Medizin, 4th edition; Urban & Fischer Verlag, Munich 1999). It is one of the fastest methods of curing in coating technology. Coating compositions based on this principle are therefore called radiation- or actinic-curing or -curable systems.
Due to the ecological and economic requirements of modern lacquer systems of using as little organic solvent as possible or even no organic solvent to adjust the viscosity, there is the desire on the one hand to use lacquer raw materials which are already low-viscosity and on the other hand moreover to be able to carry out necessary viscosity adjustments with water as the solvent.
Widely used systems of which the viscosity is adjusted by water are polyurethane dispersions which have also established themselves as radiation-curable raw materials. Nevertheless, dispersions are relatively difficult to prepare due to the necessary dispersing step, and as a rule have only a low solids content of from 30 to 50 wt. %. The high water content of these dispersions must be removed again after application and before curing. On the one hand the possible application thickness of the material is therefore reduced, and on the other hand a longer time for drying in air is required.
Water-dilutable, radiation-curable polyurethane systems with which it is possible to achieve very high solids contents of more than 80 wt. % have also been disclosed. These are as a rule polyurethanes which are based on polyethylene oxide polyols and can be diluted with a little water to give a clear solution.
Such a radiation-curable, water-dilutable polyurethane system is described, for example, in WO-A 2009/095432. The binders described can indeed be diluted with water, but after radiation curing they are very soft and not resistant to chemicals. In some cases the binders are solid at a solids content of 100 wt. % because of a high polyether content, and they must first be melted, which makes their handling expensive.
Water-dilutable, radiation-curable polyurethane systems are also described in DE-A 102010001956. The binders disclosed likewise can be diluted with water, but after radiation curing they are very soft and not resistant to chemicals.
It was therefore the object of the present invention to provide a process for the preparation of a radiation-curable polyurethane (meth)acrylate which is of particularly low viscosity as an undiluted system with a solids content of 100 wt. %, i.e. has shear viscosities at 23° C. in accordance with DIN EN ISO 3219 of ≦20 Pas, preferably ≦15 Pas and particularly preferably ≦10 Pas, and can be diluted with water over a wide range. Furthermore, the water-diluted radiation-curable polyurethane (meth)acrylate should be storage-stable. The radiation-cured films should have a high pendulum hardness and an adequate resistance to chemicals.